Thermal Reactor Synthesis of Nanoscale Ceramic Powders using Organosilazane Aerosols
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THERMAL REACTOR SYNTHESIS OF NANOSCALE CERAMIC POWDERS USING ORGANOSILAZANE AEROSOLS
Tongsan D. Xiao, Peter R. Strutt, Kenneth E. Gonsalves, & V. Shankar. The University of Connecticut, Institute of Materials Science, Department of Metallurgy, U-136, Storrs, CT 06268. ABSTRACT This investigation involves the thermal conversion of an organosilazane precursors into ultrafine ceramic particles. In this process, aerosols of a reactive organosilazane precursor, [CH 3SiHNH],, n = 3 or 4, are injected into a hot furnace to obtain Si3N4 /SiC ceramic powders. One critical feature examined in this process was the rapidity of the powder synthesis, in a reaction which involves (i) elimination of ligand groups, (ii) formation of ceramic species, and (iii) condensation of ceramic species into ultrafine ceramic particles. Accompanying studies a model has been formulated to determine the gas cooling rate and particle size. INTRODUCTION The ability to assemble molecular groups and clusters in a precise and predetermined way opens up interesting possibilities for creating materials possessing with highly homogeneous structures, which exhibit improved, and even novel properties. The importance of this concept, in the synthesis of metallic and ceramic materials, has been clearly recognized by Gleiter [I], who created what he termed nanocrystalline solids. Structurally these consist of ultrafine grains, with a mean diameter of a few nanometers. Gleiter [1] , and later Siegel [2] produced nanocrystalline materials in the form of powders by rapid vapor phase condensation of atomic species from simple evaporation sources. Consolidation. into bulk samples was achieved by high pressure compaction. We had earlier reported a technique developed for the ultra-rapid conversion of a silazane "monomeric" precursor to intermediate polymeric preceramic Si-N-C containing nanoparticles [3] . This occurred by laser-induced polycondcnsation and rapid cross-linking reactions using ultrasonic injection of a liquid precursor into the beam of a high-power industrial CO 2 laser. Strong energy coupling between the laser beam and the injected aerosol droplets resulted in their rapid evaporation and formation of a plume. Rapid condensation and cross-linking from the plume is interesting, since strong cross-linking led to the formation of intermediate precursor particles with a mean diameter of 60 nm. As an alternative to the above method we have now developed an experimental system for the generation and conversion of reactive precursors, in which an ultrasonically generated precursor aerosol was injected into a high temperature reactor tube. The end product nanoparticles, formed by rapid condensation from the vapor phase, were collected in a cooled container. Critical to the success of this continuous-flow reactor was again the design of the volatile precursors. Most of the work to-date has been done on a silazane compound, which exists as a 6 or 8 membered cyclic compound. EXPERIMENTAL The structure of the liquid silazanc precursor is shown in Fig.l. The sy
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